This invention relates to a measuring device for measuring an industrial process variable with a predetermined maximum power consumption by the measuring device. More specifically, the present invention relates to a measuring device for connection to a current loop, in particular a 4-20 ma current loop, or to a digital communication.
Devices for measuring a process variable are utilized to detect a process variable and pass the measured values on for subsequent processing. Transmission of the measured values may be effected by means of a current loop or a digital communication. In either case it is of advantage for the measuring device to draw its required power from the two lines via which the measured value is transmitted.
When the measured values are transmitted via a current loop, the current in the loop is selected so that its magnitude reflects the magnitude of the process variable. According to established standards, currents of a magnitude of between 4 ma and 20 ma are currently employed, with a current of 4 ma passing through the current loop being representative of the maximum (or minimum ) measured value, and a current of 20 ma being representative of the minimum (or maximum) measured value of the process variable.
This measurement technique has proven to be largely insusceptible to interference and has found widespread acceptance in industrial applications.
A measuring device supplied with power from a current loop has only a limited amount of power available. This power depends on the supply voltage and the particular current setting to which it is adjusted (according to the measurement value to be provided). Conventional measuring devices are dimensioned so as to make do with the minimum available power, meaning that they require only the power present at a minimum current and a minimum voltage. If more power is available, this additional power is converted into power loss in a current stage, rather than being used in the measuring device for the benefit of the measurement.
Measuring devices driven via a digital communication often have a constant current consumption which is a requirement for data transmission. Here the available power is dependent on the terminal voltage applied. Also in this technique conventional measuring devices are designed so that the measurement circuit has a constant power consumption corresponding to the power at a minimum supply voltage. Any additionally offered power at a higher supply voltage is likewise converted into power loss.
From EP 0 687 375 a suggestion for improvement is known in which an intelligent transmitter is equipped with a sensing circuit. The transmitter is operated at a measuring frequency corresponding to a power consumption exceeding the power available from the current loop at a minimum current and a minimum voltage. If a deficit results (i.e., the consumed power exceeds the permissible available power), the sensing circuit will detect this deficit and cause execution of the measurement routine to be halted until the deficit is made up.
Aside from producing other problems, this approach leads to repeated measurement errors which is not acceptable.
It is an object of the present invention to provide a measuring device of the type initially referred to which is in a position of matching its power requirements to the available power without incurring the risk of erroneous readings.
Desirably, the total amount of power consumed to perform the measuring task is an as closely as possible approximation to the amount needed to optimize speed and quality of the measurement. Theoretically, therefore, the total power which corresponds to the particular measurement value to be read would be consumed by the correspondingly frequent operation of the sensing element. In practice, however, safety reasons demand that a certain difference remain between the available power and the power consumed to perform the measuring task in order to prevent a power deficit and hence a malfunction of the sensor from occurring. The surplus of power is converted into power loss (heat) in the measuring device. The sum of the two combined power consumptions must be precisely of a magnitude causing the total current consumed by the sensor to correspond to a defined value. With the sensor this value is predetermined within a current loop (4-20 ma) by the actual measurement value to be output.
With a sensor communicating digitally, for example, the value of the constant current consumption corresponds the general specifications in connection with the communications protocol employed.
According to the invention the object is solved with the combinations of features as defined in the independent claims.
Generally, in the most preferred embodiments of the invention the desired adaptation of the power consumed for performing the measuring task to the available power without exceeding it is made possible by determining the actual surplus of power which would have to be converted into power loss. Following determination of this actual surplus, the control unit of the sensor is in a position, by making appropriate provision with respect to type and frequency of the measurement cycles performed, to approximate the power consumption of the measuring device to the predetermined maximum available power so that the surplus is minimized without falling below a predetermined limit for the surplus. (Ideally, therefore, the surplus at this limit is at least approximately equal to zero.)
Determination of the actual surplus may be effected by direct measurement of the surplus current or the surplus power. However, an indirect approach is equally possible, comprising the steps of measuring the current or consumed power for performing the measuring task and measuring the available power or using the known amount of available current, and determining the actual surplus by subtraction. When the indirect approach of surplus determination is selected, a substantial simplification incurring a minor disadvantage is achievable by dispensing with individual measurements for current or power determination, substituting therefor suitable estimations and keeping larger reserves.
Furthermore, in the determination of the power consumed for carrying out the measuring task it is often possible to limit such determination to the power consumption of those circuit elements which are known to carry most of the weight.
The present invention is suitable for any type of measuring device for process variables, provided that these measuring devices are assigned a predetermined power consumption externally, usually a varying maximum power consumption. This involves, for example, specifying the power consumption when power is supplied by a loop, because (varying with the measurement value to be indicated) only such a maximum amount of power may be consumed as corresponds to the current allowed to flow in the supply lines to provide an accurate readout.
It will be understood, of course, that the power consumption limit imposed on the measuring device may also result from other considerations as, for example, the connection with a digital communication, or for entirely different reasons.
Specifically, the present invention is particularly suited for use with sensors as, for example fluid level sensors. The present invention will be described in the following with reference to two embodiments involving a radar fluid level sensor on the one hand and an ultrasonic fluid level sensor on the other hand. Typically, such sensors are nowadays powered by current loops or digital communications (Profibus Pa., Fieldbus Foundation, . . . ), hence encountering the difficulties to be overcome according to the invention.
A preferred implementation of the invention utilizes a current stage generally connected in parallel with the remaining components of the measuring device. The current stage serves to consume the power (xe2x80x9cpower lossxe2x80x9d) that remains after subtracting the power demand of the measuring device in the measurement mode from the total power (predetermined by the measurement value readout function). As set forth previously, this non-used power surplus is a measure of the reserve available in the system for increasing the measurement performance without producing the deficit referred to in the prior art (EP 0 687 375).
Such a current stage offers a variety of possibilities of measuring the power surplus as will be explained in the following with reference to the preferred embodiments.
One such possibility comprises measuring the instantaneous power surplus directly. Alternatively, it may also be the subject of prior estimation. To do this, known data of the measuring device as, for example, the relatively high power consumption of individual components, may be referred to.
It is not always necessary to perform a continuous measurement or calculation of the continuously varying power demand. A simpler solution comprises subdividing the total range available, that is, for example, 4 to 20 ma, into sub-ranges each of which is assigned a specific frequency of measurement per unit of time. This is a very simple way of effecting measurements relatively frequently in the sub-range corresponding to the highest predetermined power consumption, whereas in those sub-ranges which correspond to lower available power, the frequency of measurement is correspondingly lower.
Then it only need be monitored in which sub-range the system is currently operating, which, for example, in the event of a 4-20 ma current loop being connected depends on which measurement value has to be output and to which current this then corresponds in order to then select the mode of operation correspondingly.
The connection of the measuring device to a digital communication or a current loop connected thereto enables completely analog arrangements to achieve the same advantages.